This invention relates to a bulk material conveying system and more particularly to such a system having improved material flow control means providing a shorter cycle time and an increased material throughput.
In the prior art, there has been developed a type of system for transporting batches of bulk material which generally consists of a pressure vessel in which the material to be transported is charged, a pneumatic conveying line communicating with a lower outlet port of the vessel for receiving material therefrom, and a gas supply system having a first branch line for supplying gas under pressure to the material conveying line for conveying material received from the vessel through the conveying line to a remote site, and a second branch line for supplying gas under pressure to an upper portion of the pressure vessel to pressurize the vessel and thus cause material to be force-fed from the vessel into the conveying line.
Typically, there is provided in such systems a shut-off valve, a manually operable flow control valve in the first branch line, and a manually operable flow control valve in the second line. In the operation of such a system, a batch of material is first loaded into the vessel. The shut-off valve is then opened to pressurize the branch lines and the flow control valve in the first line is opened and adjusted manually to provide a conveying pressure at the point of entry of the material from the vessel to the conveying line, in the range of 20 to 50 psi. The flow control valve in the second branch line is then opened and adjusted manually to provide a pressure in the upper portion of the vessel and further provide a pressure differential in the vessel in the range of 2 to 4 psi, ideally providing a flow rate of material in the conveying line of 20 to 25 ft/sec.
It has been found that such prior art systems have several disadvantages. By adjusting the valve in the second branch line to a restrictive setting sufficient to provide the desired operating pressure in the upper portion of the vessel, a prolonged time is required to pressurize the vessel thus correspondingly increasing the cycle time of the system and decreasing the material throughput. In addition, upon an overfeeding of material from the vessel into the conveying line and the possible occurrence of a partial or complete blockage of material in the conveying line, and a corresponding increase in pressure in the first branch line perhaps tending to remove such blockage, a corresponding increase in pressure develops in the upper portion of the vessel. The effect of such pressure increase in the upper portion of the vessel is to continue to feed material into the conveying line and thus compound the blockage problem. Upon removal of the blockage and a slow dissipation of the increased pressure in the conveying line, the increased pressure in the upper end of the vessel often has the effect of again overfeeding the conveying line and causing blockage to reoccur. It thus has been found to be desirable to provide a bulk material transport system of the type described having an improved flow control means which will have the effect of eliminating such disadvantages.